In recent years, liquid crystal display devices, which are thin and light in weight, are used as personal computer displays and PDA (personal digital assistance) displays. However, conventional twist nematic (TN) type and super twist nematic (STN) type liquid crystal display devices have a narrow viewing angle. Various technical developments have been undertaken to solve the problem.
A typical technique for improving the viewing angle characteristic of a TN or STN type liquid crystal display device is to add an optical compensation plate thereto. Another approach is to employ a transverse electric field mode in which a horizontal electric field with respect to the substrate plane is applied across the liquid crystal layer. Transverse electric field mode liquid crystal display devices have been attracting public attention and are mass-produced in recent years. Still another technique is to employ a DAP (deformation of vertical aligned phase) mode in which a nematic liquid crystal material having a negative dielectric anisotropy is used as a liquid crystal material and a vertical alignment film is used as an alignment film. This is a type of ECB (electrically controlled birefringence) mode, in which the transmittance is controlled by using the birefringence of liquid crystal molecules.
While the transverse electric field mode is an effective approach to improve the viewing angle, the production process thereof imposes a significantly lower production margin than that of a normal TN type device, whereby it is difficult to realize stable production of the device. This is because the display brightness or the contrast ratio is significantly influenced by variations in the gap between the substrates or a shift in the direction of the transmission axis (polarization axis) of a polarization plate with respect to the orientation axis of the liquid crystal molecules. It requires further technical developments to be able to precisely control these factors and thus to realize stable production of the device.
In order to realize a uniform display without display non-uniformity with a DAP mode liquid crystal display device, an alignment control is necessary. An alignment control can be provided by, for example, subjecting the surface of an alignment film to an alignment treatment by rubbing. However, when a vertical alignment film is subjected to a rubbing treatment, rubbing streaks are likely to appear in the displayed image, and it is not suitable for mass-production.
In view of this, the present inventor, along with others, has disclosed, in Japanese Laid-Open Patent Publication No. 2003-43525, yet another approach for performing an alignment control without a rubbing treatment, in which a plurality of openings are provided in one of a pair of electrodes opposing each other via a liquid crystal layer therebetween so that the orientation direction of the liquid crystal molecules is controlled by an inclined electric field produced at the edge portions of these openings. With this approach, a stable orientation with a sufficient degree of continuity in the orientation of the liquid crystal molecules can be obtained across the entirety of each picture element, thereby improving the viewing angle and realizing a high-quality display.
Liquid crystal display devices capable of producing a high-quality display both outdoors and indoors have been proposed in the art (see, for example, Japanese Laid-Open Patent Publication No. 11-101992). A liquid crystal display device of this type is called a “transmission-reflection type liquid crystal display device”, and includes, in each picture element region, a reflection region where an image is displayed in a reflection mode and a transmission region where an image is displayed in a transmission mode.
More recently, however, there is a demand for a further increase in the aperture ratio to produce a brighter display, in addition to the demand for an increase in the viewing angle and the display quality. No particular approach has been established in the art for further improving the aperture ratio in a case where an alignment control is done by using an inclined electric field, as disclosed in Japanese Laid-Open Patent Publication No. 2003-43525.
Moreover, optimal configurations have not yet been found for cases where the alignment control using an inclined electric field is applied to a transmission-reflection liquid crystal display device.